1. Field of the Invention
The present invention relates generally to an optical device used in a communication system in which optical fibers are utilized. More particularly, the present invention relates to an optical device having rod lenses.
2. Related Background Art
Recently, due to the explosive growth of the internet, there is a strong demand for increase in capacity of an optical fiber communications network. For the purpose of increasing the capacity, the development of WDM (Wavelength Division Multiplexing) communication has been accelerated. In the WDM communication, since lights with slightly different wavelengths convey individual pieces of information, an optical function element such as a filter, an isolator, or the like with excellent wavelength selectivity is required in an optical device.
The optical device often has a configuration such that light leaving an end face of an optical fiber from which light outgoes (hereinafter referred to as an xe2x80x9coutput fiberxe2x80x9d throughout the present specification) is changed to a parallel beam by a collimator, and the parallel beam then is allowed to pass through a flat plate component with a filter or isolator function and is condensed by a condenser lens again to enter an end face of an optical fiber from which light enters (hereinafter referred to as an xe2x80x9cinput fiberxe2x80x9d throughout the present specification). A rod lens with a refractive index distribution in its radial direction, a glass ball lens, a pressed aspherical lens, or the like is used as the collimator or the condenser lens. In view of the shape and aberration correction, the rod lens can be used most easily.
FIG. 1 is a schematic view showing an optical system of an optical device having rod lenses. Generally, as shown in FIG. 1, end faces (facing rod lenses) of an output fiber 1 and an input fiber 2 are processed to have slopes with a tilt angle of 6xc2x0 to 8xc2x0 to prevent crosstalk caused by reflected light (the tilt angle of the end face of the output fiber 1 is indicated as xcex8FA, and that of the end face of the input fiber 2 as xcex8FB). For the same reason, the end faces (facing the optical fibers) of a first rod lens 3 and a second rod lens 4 also are processed to have slopes (the tilt angle of the end face of the first rod lens 3 is indicated as xcex8PA, and that of the end face of the second rod lens 4 as xcex8PB). The output fiber 1 and the first rod lens 3 are positioned to oppose each other with a suitable air space WA being provided therebetween, and the second rod lens 4 and the input fiber 2 are positioned to oppose each other with a suitable air space WB being provided therebetween. In order to reduce the loss due to reflected light, the spaces between the output fiber 1 and the first rod lens 3 and between the second rod lens 4 and the input fiber 2 may be filled with a transparent liquid or solid with a refractive index close to that of the optical fibers and the rod lenses in some cases. An optical function element such as a filter, an isolator, or the like is placed between the rod lenses 3 and 4 (i.e. in a space L).
Generally, a single-mode fiber is used as an optical fiber for communication, and therefore a beam leaving the fiber is a Gaussian beam. In the present specification, the light ray with a highest intensity at the symmetrical center of the Gaussian beam is referred to as a xe2x80x9ccenter light rayxe2x80x9d. In order to increase the coupling efficiency between the optical fibers 1 and 2 shown in FIG. 1, the respective optical fibers 1 and 2 and rod lenses 3 and 4 are required to be positioned so that the following conditions (1) to (3) are satisfied.
(1) A beam leaving the output fiber 1 is focused at a focal point on the end face of the input fiber 2.
(2) The numerical aperture NA at the focal point is the same as that of the input fiber 2.
(3) The path of the center light ray entering the input fiber 2 coincides with the optical axis of the input fiber 2.
Furthermore, it is desirable that the respective rod lenses 3 and 4 have a numerical aperture NA at least 1.5 times to 2 times the numerical aperture NA of the optical fibers so as to transmit the Gaussian beam without causing eclipse in actual use. Naturally, various aberrations in the working wavelengths should be corrected sufficiently in the rod lenses 3 and 4.
As shown in FIG. 1, however, when the optical axes of all the optical fibers and rod lenses are aligned, it becomes difficult to satisfy the conditions described above due to the presence of many processed faces with slopes. Consequently, either or both of
(4) the shift of the focal point from the optical axis of the input fiber 2, or/and
(5) the tilt of the center light ray with respect to the optical axis of the input fiber 2
is/are caused and thus the coupling efficiency is decreased. Table 1 below shows specific design values in the case where optical axes of all the optical fibers and rod lenses are aligned and the tilt angles (xcex8FA, xcex8FB, xcex8PA, and xcex8PB) are set to be 8xc2x0 uniformly (Reference Example 1), as an example. In this case, the xe2x80x9cshift of the focal pointxe2x80x9d is very small, but the xe2x80x9ctilt of the center light rayxe2x80x9d is great, namely 2.85xc2x0, resulting in low coupling efficiency, namely 77.3% (xe2x88x921.118 dB). As shown in FIG. 13, therefore, it is required to make the optical axis of the input fiber 2 and center light ray coincide by making corrections in the input fiber 2 (correcting the tilt angle, positions in X and Y directions, and the like). Reference Example 2 (see Table 1 below) was obtained through a correction of the tilt angle in Reference Example 1. In Reference Example 2, the coupling efficiency is improved to 98.28% (xe2x88x920.075 dB).
 
The same correction also can be made by the tilting or shifting of the output fiber 1 and the respective rod lenses 3 and 4, individually.
However, it takes time to correct the positions (in the X and Y directions) and the tilt angles of the optical fibers and the rod lenses, which causes cost increase.
Therefore, in view of the assembly of an optical device, for instance, as shown in FIG. 2, it is desirable to hold the output fiber 1 and the input fiber 2 with ferrules 5 with the same outer diameter as those of the first and second rod lenses 3 and 4 and to insert the input fiber 2, the second rod lens 4, the first rod lens 3, and the output fiber 1 into a single sleeve 6, sequentially. In this case, it is not possible to carry out the xe2x80x9cposition shiftxe2x80x9d and xe2x80x9ctilt angle correctionxe2x80x9d, but it is possible to adjust the positions of the respective optical fibers 1 and 2 by pulling and inserting them in a Z axis direction. In FIG. 2, numeral 8 indicates an optical function element.
In order to obtain high coupling efficiency in the configuration shown in FIG. 2, an optical system is required, which is designed so that the above-mentioned conditions are satisfied through the adjustment of the respective optical fibers 1 and 2 in the Z axis direction alone.
With such a current situation in mind, the present invention is intended to provide an optical device including optical fibers and rod lenses with all their optical axes being allowed to coincide, particularly, through optimization of the angles of faces of the rod lenses processed to have slopes, wherein the path of a center light ray entering an input fiber is allowed to coincide with the optical axes.
In order to achieve the above-mentioned object, an optical device with a first configuration according to the present invention includes an output fiber, a first rod lens, a second rod lens, an input fiber, and an optical function element. The first rod lens converts a beam leaving an end face of the output fiber into a substantially parallel light ray, and after passing through the optical function element, the substantially parallel light ray is condensed by the second rod lens and then enters the input fiber. Optical axes of the output fiber, the first rod lens, the second rod lens, and the input fiber all coincide. The refractive index distribution of the first rod lens is expressed by
nA(r)2=n0A2xc2x7{1xe2x88x92(gAxc2x7r)2+h4A(gAxc2x7r)4+h6A(gAxc2x7r)6+h8A(gAxc2x7r)8+ . . . },xe2x80x83xe2x80x83Eq. 1
wherein r denotes a radial distance from the optical axis of the first rod lens, n0A a refractive index on the optical axis of the first rod lens, and gA, h4A, h6A, and h8A refractive index distribution coefficients. The refractive index distribution of the second rod lens is expressed by
nB(r)2=n0B2xc2x7{1xe2x88x92(gBxc2x7r)2+h4B(gBxc2x7r)4+h6B(gBxc2x7r)6+h8B(gBxc2x7r)8+ . . . },xe2x80x83xe2x80x83Eq. 2
wherein r denotes a radial distance from the optical axis of the second rod lens, n0B a refractive index on the optical axis of the second rod lens, and gB, h4B, h6B, and h8B refractive index distribution coefficients. A relationship of
xcex8FAxc2x7xcex8FB greater than 0xe2x80x83xe2x80x83Eq. 3
is satisfied, wherein xcex8FA indicates an angle between a line normal to a face, from which light outgoes, (hereinafter referred to as xe2x80x9can outgoing facexe2x80x9d) of the output fiber and an optical axis of the optical device as a whole and xcex8FB denotes an angle between a line normal to a face, from which light enters, (hereinafter referred to as xe2x80x9can incident facexe2x80x9d) of the input fiber and the optical axis of the optical device as a whole. When xcex83A and xcex83B are defined by
xcex83A=n0Axc2x7gAxc2x7WAxc2x7(nFA/(nmxc2x7nLA)+(nMxe2x88x92n0A)xc2x7xcex8QA/nMxe2x80x83xe2x80x83Eq. 4
and
xcex83B=n0Bxc2x7gBxc2x7WBxc2x7(nFBxe2x88x92nLB)xcex8FB/(nMxc2x7nLB)+(nMxe2x88x92n0B)xc2x7xcex8QB/nM,xe2x80x83xe2x80x83Eq. 5
wherein nFA indicates a core-center refractive index of the output fiber, nFB a core-center refractive index of the input fiber, WA an interval between the output fiber and the first rod lens, WB an interval between the second rod lens and the input fiber, nLA a refractive index of a medium between the output fiber and the first rod lens, nLB a refractive index of a medium between the second rod lens and the input fiber, nM a refractive index of a medium between the first rod lens and the second rod lens, xcex8QA an angle between a line normal to an outgoing face of the first rod lens and the optical axis of the optical device as a whole, and xcex8QB an angle between a line normal to an incident face of the second rod lens and the optical axis of the optical device as a whole, xcex83A and xcex83B satisfy
0xe2x89xa6|xcex83Axe2x88x92xcex83B|xe2x89xa6xcfx80/180xe2x80x83xe2x80x83Eq. 6
and relationships of
0xe2x89xa6↑xcex8PA|xe2x89xa615xc2x7(xcfx80/180)xe2x80x83xe2x80x83Eq. 7
0xe2x89xa6↑xcex8PB|15xc2x7(xcfx80n/180)xe2x80x83xe2x80x83Eq. 8
are satisfied, wherein xcex8PA denotes an angle between a line normal to an incident face of the first rod lens and the optical axis of the optical device as a whole, and xcex8PB an angle between a line normal to an outgoing face of the second rod lens and the optical axis of the optical device as a whole.
According to the optical device with the first configuration, the path of a center light ray entering the input fiber is allowed to coincide with the optical axis of the whole in the configuration in which the angles of the faces of the first and second rod lenses processed to have slopes are optimized and thus the optical axes of the output fiber, the first rod lens, the second rod lens, and the input fiber all coincide. As a result, the assembly and adjustment of the optical device can be simplified considerably, and thus the production cost can be reduced.
An optical device with a second configuration of the present invention includes an output fiber, a first rod lens, a second rod lens, an input fiber, and an optical function element. The first rod lens converts a beam leaving an end face of an output fiber into a substantially parallel light ray, and after passing through the optical function element, the substantially parallel light ray is condensed by the second rod lens and then enters the input fiber. Optical axes of the output fiber, the first rod lens, the second rod lens, and the input fiber all coincide. The refractive index distribution of the first rod lens is expressed by
nA(r)2=n0A2xc2x7{1xe2x88x92(gAr)2+h4A(gAxc2x7r)4+h6A(gAxc2x7r)6+h8A(gAxc2x7r)8+ . . . },xe2x80x83xe2x80x83Eq. 9
wherein r denotes a radial distance from the optical axis of the first rod lens, n0A a refractive index on the optical axis of the first rod lens, and gA, h4A, h6A, and h8A refractive index distribution coefficients. The refractive index distribution of the second rod lens is expressed by
nB(r)2=n0B2xc2x7{1xe2x88x92(gBxc2x7r)2+h4B(gBxc2x7r)4+h6B(gBxc2x7r)6+h8B(gBxc2x7r)+ . . . },xe2x80x83xe2x80x83Eq. 10
wherein r denotes a radial distance from the optical axis of the second rod lens, non a refractive index on the optical axis of the second rod lens, and gB, h4B, h6B, and h8B, refractive index distribution coefficients. A relationship of
xcex8FAxc2x7xcex8FB less than 0xe2x80x83xe2x80x83Eq. 11
is satisfied, wherein xcex8FA indicates an angle between a line normal to an outgoing face of the output fiber and an optical axis of the optical device as a whole and xcex8FB denotes an angle between a line normal to an incident face of the input fiber and the optical axis of the optical device as a whole. When xcex8QA0 and xcex8QB0 are defined by
xcex8QA0=n0Axc2x7gAxc2x7WAxc2x7(n0Axe2x88x92nLA)xc2x7xcex8PA/{nLAxc2x7(n0Axe2x88x92nM)}xe2x80x83xe2x80x83Eq. 12
and
xcex8QB0=n0Bxc2x7gBxc2x7WBxc2x7(n0Bxe2x88x92nLB)xc2x7xcex8PB/{nLBxc2x7(n0Bxe2x88x92nM)},xe2x80x83xe2x80x83Eq. 13
wherein if n0Axe2x88x92nM=0, xcex8QA0=0, and if n0Bxe2x88x92nM=0, xcex8QB0=0, and nFA indicates a core-center refractive index of the output fiber, nFB a core-center refractive index of the input fiber, WA an interval between the output fiber and the first rod lens, WB an interval between the second rod lens and the input fiber, nLA a refractive index of a medium between the output fiber and the first rod lens, nLB a refractive index of a medium between the second rod lens and the input fiber, nM a refractive index of a medium between the first rod lens and the second rod lens, xcex8PA an angle between a line normal to an incident face of the first rod lens and the optical axis of the optical device as a whole, and xcex8PB an angle between a line normal to an outgoing face of the second rod lens and the optical axis of the optical device as a whole, relationships of
xe2x88x92(xcfx80/180)xe2x89xa6xcex8QAxe2x88x92xcex8QA0xe2x89xa6(xcfx80/180)xe2x80x83xe2x80x83Eq. 14
and
xe2x88x92(xcfx80/180)xe2x89xa6xcex8QBxe2x88x92xcex8QB0xe2x89xa6(xcfx80/180)xe2x80x83xe2x80x83Eq. 15
are satisfied, wherein xcex8QA denotes an angle between a line normal to an outgoing face of the first rod lens and the optical axis of the optical device as a whole, and xcex8QB indicates an angle between a line normal to an incident face of the second rod lens and the optical axis of the optical device as a whole, and
xcex8PA and xcex8PB satisfy
0xe2x89xa6|xcex8PA|xe2x89xa615xc2x7(xcfx80/180)xe2x80x83xe2x80x83Eq. 16
and
0xe2x89xa6|xcex8PB|xe2x89xa615xc2x7(xcfx80/180).xe2x80x83xe2x80x83Eq. 17
According to the optical device with the second configuration, the path of a center light ray entering the input fiber is allowed to coincide with the optical axis of the whole in the configuration in which the angles of the faces of the first and second rod lenses processed to have slopes are optimized and thus the optical axes of the output fiber, the first rod lens, the second rod lens, and the input fiber all coincide. As a result, the assembly and adjustment of the optical device can be simplified considerably, and thus the production cost can be reduced.
In the optical device with the first configuration of the present invention, it is preferable that when xcex8QA0 and xcex8QB0 are defined by
xcex8QA0={(0.5xc2x7n0A2xc2x7gA2xc2x7Lxc2x7WAxe2x88x92nLAxc2x7nM)xc2x7(nFAxe2x88x92nLA)xcex8FA+nLAxc2x7nM(n0Axe2x88x92nLA)xc2x7xcex8PA}/{0.5xc2x7n0Axc2x7nLAxc2x7gAxc2x7Lxc2x7(n0Axe2x88x92nM)}xe2x80x83xe2x80x83Eq. 18
in the case of L greater than 0 and
xcex8QA0=0 in the case of L=0, and
xcex8QB0={(0.5xc2x7n0B2xc2x7gB2xc2x7Lxc2x7WBxe2x88x92nLBxc2x7nM)xc2x7(nFBxe2x88x92nLB)xcex8FB+nLBxc2x7nM(n0Bxe2x88x92nLB)xc2x7xcex8PB}/{0.5xc2x7n0Bxc2x7nLBxc2x7gBxc2x7Lxc2x7(n0Bxe2x88x92nM)}xe2x80x83xe2x80x83Eq. 19
in the case of L greater than 0 and
xcex8QB0=0 in the case of L=0,
wherein L denotes an interval between the first rod lens and the second rod lens on the optical axis of the optical device as a whole, a relationship of
xe2x88x922.5xc2x7(xcfx80/180)xe2x89xa6(xcex8QA+xcex8QB)xe2x88x92(xcex8QA0+xcex8QB0)xe2x89xa6+2.5xc2x7(xcfx80/180)xe2x80x83xe2x80x83Eq. 20
is satisfied, wherein xcex8QA denotes an angle between a line normal to an outgoing face of the first rod lens and the optical axis of the optical device as a whole, and xcex8QB indicates an angle between a line normal to an incident face of the second rod lens and the optical axis of the optical device as a whole.
In the optical devices with the first and second configurations of the present invention, it is preferable that a path of a light ray at a symmetrical center of light intensity distribution of a beam leaving the output fiber coincides with the optical axis of the optical device as a whole after the light ray enters the input fiber.
In the optical devices with the first and second configurations of the present invention, it is preferable that a relationship of 1.4xe2x89xa6n0Axe2x89xa62.0 is satisfied.
In the optical devices with the first and second configurations of the present invention, it is preferable that a relationship of 1.4xe2x89xa6n0Bxe2x89xa62.0 is satisfied.
In the optical devices with the first and second configurations of the present invention, it is preferable that a relationship of 0.125 mmxe2x89xa62 r0Axe2x89xa65 mm is satisfied, wherein r0A indicates a radius of the first rod lens.
In the optical devices with the first and second configurations of the present invention, it is preferable that a relationship of 0.125 mmxe2x89xa62 r0Bxe2x89xa65 mm is satisfied, wherein r0B indicates a radius of the second rod lens.
In the optical devices with the first and second configurations of the present invention, it is preferable that a relationship of 0.1xe2x89xa6n0Axc2x7gAxc2x7r0Axe2x89xa61 is satisfied.
In the optical devices with the first and second configurations of the present invention, it is preferable that a relationship of 0.1xe2x89xa6n0Bxc2x7gBxc2x7r0Bxe2x89xa61 is satisfied.
In the optical devices with the first and second configurations of the present invention, it is preferable that a relationship of 4xc2x7(xcfx80/180)xe2x89xa6|xcex8FA|xe2x89xa615xc2x7(xcfx80/180) is satisfied, and further preferably, a relationship of 6xc2x7(xcfx80/180) xe2x89xa6|xcex8FA|xe2x89xa68xc2x7(xcfx80/180) is satisfied.
In the optical devices with the first and second configurations of the present invention, it is preferable that a relationship of 4xc2x7(xcfx80/180)xe2x89xa6|xcex8FB|xe2x89xa615xc2x7(xcfx80/180) is satisfied, and further preferably, a relationship of 6xc2x7(xcfx80/180) xe2x89xa6|xcex8FB|xe2x89xa68xc2x7(xcfx80/180) is satisfied.
In the optical devices with the first and second configurations of the present invention, it is preferable that a relationship of gAxc2x7WAxe2x89xa60.2 is satisfied.
In the optical devices with the first and second configurations of the present invention, it is preferable that a relationship of gBxc2x7WBxe2x89xa60.2 is satisfied.
In the optical devices with the first and second configurations of the present invention, it is preferable that a relationship of |xcex8FA|=|xcex8FB| is satisfied.
In the optical devices with the first and second configurations of the present invention, it is preferable that a relationship of nLA=nLB is satisfied, and further preferably, a relationship of nLA=nLB=1 is satisfied.
In the optical devices with the first and second configurations of the present invention, it is preferable that relationships of n0A=n0B, gA=gB, and r0A=r0B are satisfied.
In the optical device with the first configuration of the present invention, it is preferable that a relationship of xcex8PA=xcex8PB is satisfied.
In the optical device with the second configuration of the present invention, it is preferable that a relationship of xcex8PA=xe2x88x92xcex8PB is satisfied.
In the optical devices with the first and second configurations of the present invention, it is preferable that a relationship of nFA=nFB is satisfied.
In the optical device with the second configuration of the present invention, it is preferable that a relationship of xcex8PA=xcex8PB=0 is satisfied.
In the optical device with the second configuration of the present invention, it is preferable that relationships of xcex8FA=xcex8PA and xcex8FB=xcex8PB are satisfied.